How to Form Concrete Footings for Garden Structures

Start With the Load, Not the Spade

Every footing begins with a simple question: what is it going to carry? A 6x4 timber shed sitting on a level patio needs far less than a brick garden room with a tiled roof, and the difference in cost between the two is measured in bags of ballast, not hundreds of pounds. So before you mark anything out, work out roughly what the structure will weigh once it is finished and loaded — including the stuff you will inevitably store inside it.

As a rule of thumb for UK garden work:

• Light structures — small timber sheds, pergolas, summerhouses, bin stores and bike shelters: strip footings 300mm wide and 150mm deep, or a simple 100mm mass concrete pad.
• Medium structures — larger sheds, timber-framed garden offices, decking posts and car ports: footings 400mm wide and 200mm deep, with a minimum of 100mm of concrete below any masonry.
• Heavy structures — brick or block garden rooms, single-storey extensions, retaining walls and anything with a tiled or felted roof: 450–600mm wide, taken down to firm subsoil, and ideally checked by someone who understands the local ground.

If you are anywhere near a boundary, a drain, a sewer or a mature tree, stop and check before you dig. Tree roots draw moisture out of shrinkable clay, and a footing poured in the wrong place can move for years afterwards.

Setting Out Square and Level

Accuracy here saves you hours later. Use builders' lines and pegs, or timber profiles for anything with corners. To get a true right angle, use the 3-4-5 method: measure 3m along one line, 4m along the other, and adjust until the diagonal between the two points is exactly 5m. Work in metres rather than feet and inches — the arithmetic is far kinder.

Check your diagonals on any rectangular layout: corner to corner both ways, and they should match within a few millimetres. Once the strings are set, mark the ground with a line of dry sand or a can of marking paint. Drop a spirit level on the strings to confirm they are level; if the ground is sloping, set the strings level and simply dig deeper at the low end so the finished footing is uniform.

Write your depths on a piece of paper before you start. It is surprisingly easy to lose track of which corner was dug to what, and a 50mm variation across a shed base is enough to cause problems with door alignment.

Digging to Firm Ground

Dig with a spade for the edges and a mattock or trenching shovel for the bulk. Keep the sides reasonably vertical and the bottom flat. The critical rule is this: the bottom of the trench must be undisturbed, firm ground. If you hit soft spots — old ash, builders' rubble, a filled pond, a run of roots — keep going until you reach something solid, then fill the extra depth with concrete. Concrete is cheap; subsidence is not.

In most UK gardens you will find topsoil for the first 200–300mm, then clay, sand, chalk or gravel. All of these are generally fine to build on. What you are looking for is consistency: if one end of the trench is on stiff clay and the other on loose fill, you have a problem and should dig deeper at the soft end.

Line the trench with a thin layer of compacted hardcore only if the ground is a bit soft but not actually unsuitable. Otherwise pour straight onto the soil. Remove any loose material and give the base a firm tamp with the end of a length of timber.

Formwork, Reinforcement and Depth

For simple strip footings, the ground itself is often enough formwork — just keep the trench edges straight. Where you need a raised or decorative edge, box it out with 18mm ply or scaffold boards, braced with pegs every 600mm so it does not bow under the weight of wet concrete.

Reinforcement is worthwhile on almost anything structural. A single layer of A142 mesh, laid on 40mm plastic spacers so it sits in the middle of the concrete, will resist the cracking that comes from ground movement and thermal shrinkage. For small sheds and pergolas it is optional but cheap insurance; for extensions, garden rooms and anything over 4m long, treat it as essential.

Add a minimum of 100mm of concrete beneath any brick or blockwork, and aim for a finished footing at least 450mm below ground level on an external wall — deeper in clay, where 750mm or more is common. Building control will want to see the depth for anything that needs approval.

Mixing and Pouring

Use a standard C20/25 mix: one part cement to four parts all-in ballast by volume, or order ready-mix for anything over a cubic metre. That is roughly one 25kg bag of cement to 100kg of ballast. Add just enough water to make it workable — a stiff mix that holds its shape is stronger than a sloppy one.

Pour in one continuous operation if you can, working from one end and tamping as you go with a length of timber to knock out air pockets. If you must stop, finish at a vertical joint rather than a sloping one, and roughen the surface before you restart.

Level the top with a straight edge, then leave it alone. Concrete needs to cure, not dry: cover it with polythene or hessian for the first few days in warm weather, and keep it damp. Do not build on it for at least 48 hours in mild conditions, and give it a full seven days before loading anything heavy.

A Few Last Checks

• Confirm your levels with a laser or a long spirit level before the concrete goes off — you cannot adjust it afterwards.
• Keep footings at least 2.5m from any large tree, and further for poplar, willow or oak.
• Sleeve any services in ducting and photograph their positions before you pour.
• Check drainage runs: a footing crossing a sewer needs a lintel or a re-route, and permission from the water authority.

Get the footings right and everything above them behaves. Get them wrong and you will spend years patching cracks, shimming doors and wondering what is happening under the floor.